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1.4 RIP
1.4.1 Introduction to RIP
RIP is first introduced in ARPANET, this is a protocol dedicated to small, simple networks.
RIP is a distance vector routing protocol based on the Bellman-Ford algorithm. Network devices
running vector routing protocol send 2 kind of information to the neighboring devices regularly:
Number of hops to reach the destination network, or metrics to use or number of networks to
pass.
What is the next hop, or the director (vector) to use to reach the destination network.
The distance vector Layer 3 switch send all their route selecting tables to the neighbor layer3
switches at regular interval. A layer3 switch will build their own route selecting information table
based on the information received from the neighbor layer3 switches. Then, it will send this
information to its own neighbor layer3 switches. As a result, the route selection table is built on
second hand information, route beyond 15 hops will be deemed as unreachable.
RIP protocol is an optional routing protocol based on UDP. Hosts using RIP send and receive
packets on UDP port 520. All layer3 switches running RIP send their route table to all neighbor
layer3 switches every 30 seconds for update. If no information from the partner is received in
180 seconds, then the device is deemed to have failed and the network connected to that device
is considered to be unreachable. However, the route of that layer3 switch will be kept in the route
table for another 120 seconds before deletion.
As layer3 switches running RIP built route table with second hand information, infinite count
may occur. For a network running RIP routing protocol, when an RIP route becomes
unreachable, the neighboring RIP layer3 switch will not send route update packets at once,
instead, it waits until the update interval timeout (every 30 seconds) and sends the update
packets containing that route. If before it receives the updated packet, its neighbors send
packets containing the information about the failed neighbor, “infinite count” will be resulted. In
other words, the route of unreachable layer3 switch will be selected with the metrics increasing
progressively. This greatly affects the route selection and route aggregation time.
To prevent “infinite count”, RIP provides mechanism such as “split horizon” and “triggered
update” to solve route loop. “Split horizon” is done by avoiding sending to a gateway routes
leaned from that gateway. There are two split horizon methods: “simple split horizon” and
“poison reverse split horizon”. Simple split horizon deletes from the route to be sent to the
neighbor gateways the routes learnt from the neighbor gateways; poison reverse split horizon
not only deletes the abovementioned routes, but set the costs of those routes to infinite.
“Triggering update” mechanism defines whenever route metric changed by the gateway, the
gateway advertise the update packets immediately, regardless of the 30 second update timer